Methods (refer to Japanese Unexamined Patent Publication No. 2010-2218 (Patent Document 1), and Japanese Unexamined Patent Publication No. 2011-7590 (Patent Document 2) for example) are conventionally known in which a terahertz detector (e.g., photoconductive switch) receives terahertz light A (which is a pulse) that is provided by a terahertz generator (for example, photoconductive switch) to an object under measurement and passes through the object under measurement, and light B of a pulse period slightly different from the pulse period of the terahertz light A, and then the detector measures the object under measurement.
In the above described related arts, a master laser provides a master laser light pulse to the terahertz generator, while a slave laser provides a slave laser light pulse (light B) to the terahertz detector. A repetition frequency of the master laser light pulse, however, is slightly deferent from that of the slave laser light pulse. Note that a technique that causes the repetition frequency of the master laser light pulse to slightly differ from that of the slave laser light pulse is described in Japanese Patent No. 4782889 (Patent Document 3) and Japanese Patent No. 4786767 (Patent Document 4).
Furthermore, a trigger signal is generated on the basis of the master laser light pulse and the slave laser light pulse. The trigger signal serves as a time origin point of a signal detected using the terahertz detector (refer to, for example, Patent Document 1, FIG. 6).